1. Field of the Invention
The present invention relates generally to the field of manufacture of oxygen-scavenging packaging materials. Particularly, it concerns the preparation of masterbatches of metal oleate, especially cobalt oleate, that avoid handling and processing problems found in previous preparative techniques.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking. In the food packaging industry, several means for limiting oxygen exposure have already been developed, including modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging. In the first two instances, reduced oxygen environments are employed in the packaging, while in the latter instance, oxygen is physically prevented from entering the packaging environment.
Another, more recent, technique for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can scavenge oxygen present inside the package. The oxygen thus scavenged either can be present in the interior when product is filled into the package, or can migrate into the package after product is filled. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it passes through the walls of the package (herein referred to as an xe2x80x9cactive oxygen barrierxe2x80x9d), thereby maintaining the lowest possible oxygen level throughout the package.
In many cases, however, the onset of oxygen scavenging in this system may not occur for days or weeks. The delay before the onset of useful oxygen scavenging is hereinafter referred to as the induction period. In addition, the rate of oxygen scavenging may also be relatively low. Much work has been done both to minimize the induction period and increase the scavenging rate. One common approach that is useful in both areas is the use of metal oleates, such as cobalt oleate, as a catalyst for oxygen scavenging.
Typically, in production of an oxygen scavenging packaging article, metal oleate is provided as part of a masterbatch with a polymer, i.e. the metal oleate is in a concentrated formulation that is added to the bulk polymer during production to generate a desired, lower concentration of the metal in the oxygen scavenging layer of the oxygen scavenging packaging article. However, metal oleates pose a number of difficulties in processing.
Neat metal oleates, such as cobalt oleate, are tacky solids at room temperature. As a result, they have tar-like properties and are difficult to handle, as pellets of metal oleate will adhere upon storage. Typically, to overcome this limitation, a metal oleate used in masterbatch production is usually provided in an organic solvent, such as in a 50% solution in toluene. This raises the price over that of, for example, a neat formulation of a metal stearate (a non-tacky solid at room temperature). Also, the organic solvent such as toluene must be removed during production of the masterbatch, requiring further equipment, and poses disposal concerns, especially for toluene, which is widely held to be carcinogenic.
Therefore, it is desirable to formulate a metal oleate masterbatch using a stock of metal oleate that can be readily handled in solid form. It is also desirable for such a stock to be free or substantially free of toluene or other solvent.
The present invention is directed to metal oleate masterbatch compositions useful in preparing oxygen scavenging packaging articles, as well as methods of making the masterbatch compositions.
In one embodiment, the masterbatch composition comprises porous beads comprising a first polymer; metal oleate dispersed in the porous beads; and a second polymer or a photoinitiator, in powder form, dispersed on the exteriors of the porous beads. Such a masterbatch composition can be prepared by a method comprising (i) providing a solution of metal oleate in an organic solvent; (ii) dispersing the solution into the beads, to form solvated metal oleate beads; (iii) evaporating the solvent from the solvated metal oleate beads, to form metal oleate dispersed in the beads; and (iv) mixing the polymer or the photoinitiator, in powder form, with the beads, to form the composition.
In another embodiment, the masterbatch composition comprises a metal oleate and a polymer, wherein the metal oleate is in the form of pellets, and the polymer is in the form of a powder dispersed on the exteriors of the pellets. Such a masterbatch composition can be prepared by a method comprising (i) providing the metal oleate in the form of pellets, and (ii) dispersing the polymer in the form of a powder on the exteriors of the pellets.
In yet another embodiment, the masterbatch composition comprises a metal oleate and a polymer, wherein the metal oleate and the polymer are compounded. Such a masterbatch can be prepared by a method comprising (i) providing the metal oleate in the form of pellets and providing the polymer as a solid and (ii) compounding the metal oleate and the polymer.
In still another embodiment, the masterbatch composition is prepared from a composition comprising a metal oleate and a polymer, wherein the polymer is in the form of a container, such as a bag, and the metal oleate is contained within the container. The composition is processed to form a masterbatch, or directly into an oxygen scavenging component of an oxygen scavenging packaging article, by the method of (i) providing the composition comprising a metal oleate and a polymer, wherein at least some of the polymer is in the form of a container and the metal oleate is contained within the container, (ii) melting the composition to produce a melted composition, and (iii) mixing the melted composition to produce a masterbatch. If the metal oleate in the container is congealed during or before the providing step, the method further comprises grinding the composition, before or during the heating step.